[llvm-dev] Redundant load in llvm's codegen compares to gcc when accessing escaped pointer?

[Markus Trippelsdorf via llvm-dev]

On 2016.03.17 at 16:35 -0700, Chris Lattner via llvm-dev wrote:
> 
> > On Mar 15, 2016, at 7:58 AM, Chuang-Yu Cheng via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> > 
> > Hi,
> > 
> > Please look at this c code:
> > 
> > typedef struct _PB {
> >   void* data;  /* required.*/
> >   int   f1_;
> >   float f2_;
> > } PB;
> > 
> > PB** bar(PB** t);
> > 
> > void qux(PB* c) {
> >   bar(&c);              /* c is escaped because of bar */
> >   c->f1_ = 0;
> >   c->f2_ = 0.f;
> > }
> > 
> > // gcc-5.2.1 with -fno-strict-aliasing -O2 on x86
> > call    bar
> > movq    8(%rsp), %rax
> > movl    $0, 8(%rax)
> > movl    $0x00000000, 12(%rax)
> > 
> > // llvm 3.9.0 with -fno-strict-aliasing -O2 on x86
> > callq    bar
> > movq    (%rsp), %rax
> > movl    $0, 8(%rax)
> > movq    (%rsp), %rax
> > movl    $0, 12(%rax)
> > 
> > You can see that llvm load "c" twice, but gcc only load "c" once.
> > Of course, in bar function, you may do something very dangerous, e.g.
> > 
> > PB** bar(PB** t) {
> >    *t = (PB*) t;
> > }
> > 
> > But gcc doesn't care bar's definition.
> > Is llvm too conservative, or gcc too aggressive in this pattern?
> 
> In my opinion, in the face of -fno-strict-aliasing, GCC is being too
> aggressive.  It would be interesting to hear what they think.

We discussed this issue briefly on the #gcc IRC channel. 
Richard Biener pointed out that bar cannot make c point to &c - 8,
because computing that pointer would be invalid. So c->f1_ cannot
clobber c itself.

-- 
Markus